Process for removing the colour from oriented monocrystals



United States Patent US. Cl. 2s2 3o1.1 7 Claims Int. Cl. 009k 1/00 ABSTRACT OF THE DISCLOSURE The present disclosure is directed to a process for the modification of coloration of perovskite crystals having the formula ALnO in which A is a trivalent ion and Ln is a trivalent rare earth ion, wherein ions having a valency greater than 3 are substituted for some of the ions A and/or Ln and then these substituting ions are oxidized to their highest valence state. By using a suitable number of ions of rare gas electronic configuration and with a valency greater than 3 it is possible to eliminate color centers due to the presence of gaps or vacancies.

The present invention concerns a new method for carrying out the removal of colour from perovskite crystals.

Copending US. application No. 299,833 of Aug. 5, 1963 in the name of the Forrat, describes processes for the manufacture of, for removal of colour from, and for the colouring of monocrystals of per'ovskites of the chemical formula:

ALI103 in which A is the generic term to designate the trivalent ions, Ti V, Cr, Al Ga, etc., and Ln is the generic term to designate trivalent rare earths.

It is known that perovskites have a crystalline structure in which the ions O++ are particularly stable on sites derived from the face-centred cubic lattice.

For this reason it is possible to obtain non-stoichiometric crystals ALn O and x being near 1, but these crystals are in general coloured.

A general study of the colour of monocrystals of the perovskites has shown that this was due either to the presence of V centres which correspond to an absence of a rare earth atom, or to the presence of F centres which correspond to an absence of an oxygen atom, or to the simultaneous presence of F and V centres.

It is known in actual fact that when the structure of the monocrystal is such that it lacks an ion, there are in the crystal a number of electrons or of available holes equal to the electronic valency of the ion. These electrons or available holes are called centres, and vibrate at luminous frequencies, which has the effect of bringing about colour in the crystal.

The result is that to make a monocrystal colourless, all that is necessary to do is to eliminate these centres.

In accordance with US. patent application No. 299,833 of Aug. 5, 1963 a method of eliminating the centres consists in making a diffusion of positive ions,'for example hydrogen ions, by means of a gaseous diffusion in the crystal at suitable temperature, for example a hydrogen diffusion.

Again in accordance with US. patent application No. 299,833 of Aug. 5, 1963, another method of eliminating these centres consists in using, in the course of the formation of the monocrystal, a slight excess of oxide of rare earth, the result being that one obtains a crystal 3,420,780 Patented Jan. 7, 1969 "ice the composition of which is stoichi-ometric, such a crystal is a colourless crystal.

Also known from the above-mentioned patent application is a process for colouring a perovskite crystal which consists in the doping by transition metal ions selected from among the sulphates or nitrates of Cr, Mn, Fe, Co, U or oxides -or salts of Ti, Nb, Th, or of salts of rare earths. The doping ions which are added have the property of showing visible electronic transitions; the result is that their corresponding absorption band is added to that of the perovskite crystal, thus bringing about a modification in colour of this latter.

The present invention describes the process by means of which it is possible to obtain the removal of colour from perovskite crystals.

The invention consists in a process for removing the colour from monocrystals and ceramics of perovskites in accordance with the chemical formula ALnO A being the generic term to designate the trivalent ions Ti V, Cr, A1 Ga, and Ln being the generic term to designate the trivalent rare earths, and having coloured centres, in which process the said centres are annihilated, characterised by the fact that on the sites of the ions A, and Ln are placed ions A and Ln of valency (V) greater than 3, which ions do not have visible absorption bands, the number y of the said ions A and Ln being shown by the formula and NL representing the number of coloured centres. The perovskites are obtained from a homogenized powder allowed to crystallize out from solution by the application of Verneuils method. The Verneuils method is a process in which powdered alumina which is mixed with a small portion of chromium (III) oxide is fused in oxyhydrogen flame, the fused mass being allowed to crystallize as a boule on an alumina rod.

Such ions, not having visible absorption bands, do not bring about modification of colour. However, by placing such an ion of valency greater than 3 on the site of the ion A or Ln this ion would be able to give up at least one electron which will remove a colour centre. By this fact, and by using a suitable number of ions, of rare gas electronic configuration and with a valency greater than 3, it will be possible to annihilate all the coloured centres thus to bring about complete removal of colour. In accordance with the present invention the said ions A and Ln are ions with rare gas configuration, and in particular when placed on the sites of the ions A; are Zr Hi and when placed on the sites of the ions Ln is Th.

In accordance with the present invention the ions A used are polyvalent transition metal ions, in particular Ti, V Nb Ta, Mo Mn, W, Ru, Re, In.

In accordance with the present invention the Ln ions are polyvalent ions of the lanthanides or actinides in particular Ce Tb However, certain of these polyvalent transition metal ions such as Mn, Ru, Re, Ir, Ce Tb, have the disadvantage of having absorption bands near the visible which although less intense than those of the V centres, for examples can be restricting.

In accordance with the present invention, the method used to remove the centres bringing about the colour consists firstly in doping the perovskite powder before crystallisation with ions previously cited, and secondly in oxidising these ions to their maximum valency level.

The doping can be efiected either from a solution of 3 their salts for the additional ions V Mo, W Mn, Ru Re Ir, Ce Th, or by mixing the oxides in the course of homogenisation which precedes the crystalline growth, for the ions Ti Zr Hf, Nb Ta Th The oxidation operation can consist in a re-cooking of the crystal or of the ceramic in the air or in oxygen atmosphere between 5-00" and 1,500" C.

It should be understood that various amendments and modifications can be brought about in the method of carrying out the invention, given simply by way of illustration, without thereby going beyond the scope of the present invention.

What we claim is:

1. A process for the modification of coloration of perovskite crystals, having the formula ALnO wherein A is a member selected from the group consisting of the trivalent ions Ti V, Cr, A1 Ga and Ln is a member selected from the group consisting of the trivalent rare earth ions said perovskites possessing ion vacancies and being obtained from a homogenized powder allowed to crystallize out from solution by the application of Verneuils method, which comprises doping the perovskite powder with ions having a valency greater than 3 and having few or no absorption bands in the visible spectrum, before crystallization of said powder, and subsequently oxidising these ions to their highest valency state.

2. Process according to claim 1 wherein the number, Y, of ions placed on the sites A and/or Ln is represented by the formula wherein NL represents the number of ion vacancies and V the highest valency of the doping ions.

3. Process according to claim 1 wherein the doping ions are selected from the group consisting of Zr, Hf 11 4+ "54+ 5+ 5+ u w W6+ 4+ m Ru, IIH', Co and Tb.

4. Process according to claim 3 wherein doping is effected from a solution of appropriate salts yielding one or more of the ions selected from the group consisting of V, [Mo Mn, Ru, Ir Ce, Th, Re and W 5. Process according to claim 3 wherein doping is effected by mixing appropriate oxides with the homogenized powder, said oxides yielding one or more of the ions selected from the group consisting of Ti, Zr Hf Nb Ta and Th.

6. Process according to claim 1 wherein the oxidation step takes place during the application of Verneuils method.

7. Process according to claim 1 wherein the oxidation step follows the application of Verneuils method and comprises re-heating the crystal in an oxygen-containing atmopshere at a temperature of about 500 and 1500 C.

References Cited UNITED STATES PATENTS 3,257,327 6/1966 Nassau 252-3015 BENJAMIN R. PADGETT, Primary Examiner.

S. J. LECHERT, JR, Assistant Examiner.

US. Cl. X.R. 

